Types of Freshwater Ecosystem Services
The goods and services provided to humans by freshwater benthic ecosystems may be classed as provisioning services, or products obtained from ecosystems, such as plant and animal food and fiber; supporting services, or services necessary for the production of all other ecosystem services, such as waste processing, the production of a sustained clean water supply, flood abatement, and climate moderation; and cultural services, or nonmaterial benefits obtained from ecosystems, such as aesthetics, education, and recreation (Millennium Ecosystem Assessment 2003). Besides natural waste treatment that enhances water quality, many freshwater ecosystems are critical habitats for certain life stages of marine and freshwater fishes, waterfowl, and other sources of human foods. Moreover, these benthic ecosystems provide critical habitat for many other species.
Understanding natural processes that contribute to ecosystem services is of immediate concern given the rates at which human activities are altering natural fresh waters.
As human population densities increase further and new chemical compounds and technologies are developed, unanticipated consequences will have long-lasting impacts on freshwater benthic ecosystems (Malmqvist & Rundle 2002). If fresh waters are degraded under intensive exploitation, their natural processes can be diminished or lost completely. As native species are lost through local extinction and nonnative species are introduced into fresh waters, there is lively debate regarding trade-offs among different management alternatives. It is critical that decision makers understand how species can provide unique roles in cycling nutrients and in producing valuable commodities and services in many different types of fresh waters. Tables 3.1a-3.1e highlight the major goods and services under a number of major categories (including food production, water quality and quantity, waste disposal, climate modification, and recreation). The relative importance of services varies across the different freshwater ecosystems.
Provisioning Services. Groundwater supplies drinking, municipal, industrial, and irrigation water worldwide. The most important ecosystem service humans receive from groundwater is providing clean water for drinking. Although abiotic processes control water quantity through recharge, microbes are especially important in producing clean water. Microbial remediation of contaminated groundwater is another ecosystem service provided by sediment-dwelling organisms. Bioremediation of groundwater often benefits from injecting microbial assemblages into contaminated sites and encouraging bacterial growth with nutrient additions (Ghiorse & Wilson 1988; Baker & Herson 1994). For example, bacteria can remove nitrate or degrade recalcitrant organic contaminants. Groundwater supports a rich food web consisting of microbes and metazoan consumers (Marmonier et al. 1993). Because these groundwater species respond to chemical contamination, they can be used to identify polluted aquifers (Gounot 1994; Moeszlacher 2000). For example, the presence of certain flagellates and their grazing of bacteria may increase degradation rates of toluene (Mattison & Harayama 2001). However, little is known about the details of food web interactions in groundwater or subsurface flows in stream channels (hyporheic zones) that alter degradation and removal rates of contaminants (e.g., nitrate, organic compounds).
Lakes and Rivers
Provisioning Services. Benthic organisms in lakes and rivers provide food production mostly through the dependence of fish production on invertebrate prey and nutrient cycling. Globally about 8 X 106 tons of freshwater fish are harvested, with double that amount produced by aquaculture (FAO 1995). Productivity of these fisheries will, in part, depend upon benthic production directly (e.g., consumption of benthic invertebrates or aquatic plants) or indirectly (e.g., benthic mineralization of nutrients). For example, Chinese polyculture relies on benthic productivity for plant and mussel production for feeding carp. Productivity of deepwater ecosystems is often influenced by how tightly the upper waters are linked to nutrient cycling in the lower waters that are in contact with sedimentary sources of nutrients. This process of pelagic-benthic coupling is critical in determining how nutrients (or toxins) are stored in sediments and seasonally cycled into surface waters, where they are incorporated into algal production and then consumed by filter-feeding zooplankton and fishes. Several marine fisheries influence and depend upon production in freshwater ecosystems (e.g., anadromous salmon spawn and juveniles are reared in freshwater rivers and lakes).
Support Services. Benthic species maintain water quality via transformation of excess nutrients and organic pollutants. For example, stream organisms can rapidly take up and incorporate nitrogen into their biomass or produce ammonia or methane that enters the atmosphere, thereby lowering the loads of dissolved organic nitrogen (Alexander et al. 2000). Nutrients such as phosphorus in streams can be temporarily stored in sediments and the biota (Meyer & Likens 1979). Benthic bacteria permanently remove nitrogen via denitrification (e.g., Pind et al. 1997) as well as convert nitrogen from unusable to usable forms that can be taken up during plant growth. Invertebrates and microbes that are widely distributed in natural ecosystems also occur in sediments and biofilms found in water-treatment plants. Thus, the biological functions are similar, although the species and densities generally vary greatly between natural and artificial habitats and these communities respond primarily to nutrient loading (e.g., Kadlec & Knight 1996; DeBruyn & Rasmussen 2002). Other species of macroinvertebrates (such as stoneflies and caddisflies) that break down particular types of organic materials are restricted in their distributions. Many occur in pristine, unmanaged habitats where low levels of nutrients and high concentrations of dissolved oxygen are sustained by a diverse assemblage of plants and animals.
The breakdown of dead organic matter (detritus) is an ecosystem service provided by most freshwater benthic communities. The roles of benthic detritivores that transform and transfer nutrients are well documented (Wallace et al. 1996; Giller & Malmqvist 1998). Benthic organisms shred coarse sizes of organic matter into finer particles. Microbial species condition detritus, which facilitates use by the shredding invertebrates, and also decompose organic particles. Microbes also produce gases (CO2, CH4, N2) that enter the atmosphere and dissolved forms of nutrients that enter the overlying waters. Dissolved nutrients increase the growth of algae and aquatic macrophytes, which in turn are consumed by herbivorous and omnivorous invertebrates and fishes, thus creating the basis for complex food webs (Covich et al. 1999; Crowl et al. 2001; Jonsson & Malmqvist 2003).
As previously discussed, water quality is maintained by a number of biotic processes that are associated with sedimentary habitats where benthic invertebrates play well-defined roles in ecosystem processes. The loss of certain species or their changes in abundance may impair ecosystem function and consequently ecosystem services. For exam ple, findings from Coweeta Hydrologic Laboratory, a US National Science Foundation Long Term Ecological Research site in North Carolina, indicated that measures of stream water quality (associated with rates of detrital processing) declined when stream insects were experimentally eliminated from a stream. Sequential declines in aquatic insect biodiversity correlated with the changes in stream ecosystem processes. This study was the first field experiment to show that these measures of water quality correlated with ecosystem processes (Wallace et al. 1996). This research also showed specifically how physical and chemical impacts (which deplete invertebrate populations) may feed back to alter stream ecosystem processes. The ecosystem-scale evidence for this linkage in streams and rivers obtained from the research at Coweeta provided detailed information about specific ways in which ecosystem-level processes change following invertebrate removal. In these studies, many species of leaf-shredding invertebrates were known to process coarse leaf-litter inputs from riparian zones into smaller particles. To test the importance of this role of shredders in ecosystem function, the Coweeta researchers experimentally removed most stream-dwelling insects using low doses of insecticide, which lowered shredder secondary production to 25 percent of that of a nearby reference stream (Lugthart & Wallace 1992). Organic carbon export from the watershed decreased dramatically following the insecticide treatment (Cuffney et al. 1990). Leaf decomposition was twice as slow in the invertebrate removal stream, so standing stocks of leaf litter were much higher. In general, lower export of organic carbon from headwater streams may lower animal production in downstream food webs, where filter-feeding species may be facilitated by shredding species living upstream (Heard & Richardson 1995).
Studies at the Luquillo Experimental Forest in Puerto Rico (another US National Science Foundation Long Term Ecological Research site) further demonstrate the potential for a single species to have an impact on ecological functions. A freshwater shrimp (Xiphocaris elongata) is one of the few species of shredders that facilitates the uptake of suspended organic particulates by a filter-feeding species of shrimp (Atya lanipes), which co-occurs in some tropical headwater streams (Crowl et al. 2001). Loss of these species' functions of shredding and filter feeding would likely result in slower rates of leaf litter breakdown and less energy flow in the headwater food web. This loss of species that shred leaf detritus may be critical in tropical headwaters, where species of shredding insects are relatively rare and the functional redundancy among leaf shredders is relatively low (Covich et al. 1999; Dobson et al. 2002). Related research is beginning to identify the degree to which ecosystem services of rivers and other freshwater ecosystems are altered directly by physical and chemical impacts (e.g., low O2, low pH, or high sedimentation) compared with being altered indirectly through the loss of key animal taxa (Jonsson & Malmqvist 2003).
Accumulation of organic matter can slow decomposition by microbial species and detritivorous invertebrates when dissolved oxygen is depleted by high rates of respiration, especially at warm temperatures. Deoxygenation subsequently results in displace ment of numerous species that require high oxygen concentrations and replacement by other species that can tolerate the stressful conditions of low dissolved oxygen. This sequence of species substitutions typically results in a degraded stream community with nuisance and disease-transmitting characteristics as well as reduced capacity for providing critical ecosystem functions. For example, heavy pollution and deoxygenation in some urbanized streams around Rio de Janeiro eliminated Atyid shrimp, which previously filtered out suspended organic matter. Following this pollution and loss of freshwater shrimp in the streams, the increased densities of filter-feeding blackflies led to more biting insects due to the loss of the same ecological function (filtration of suspended organic matter) by the shrimp (Moulton 1999). Other benthic invertebrates directly serve as biocontrol agents by feeding upon vectors of diseases (e.g., aquatic insects and crustaceans that feed on certain species of mosquito larvae and snails) that are prevalent in tropical freshwater habitats. Field studies substantiate the widespread importance of benthic invertebrates as indicators of water quality and as functional regulators of important ecosystem functioning (Clements & Newman 2002).
Wetlands and Associated Freshwater Habitats
Among the most critical and scarce freshwater ecosystems are marshes, floodplains, and swamps. Although they cover only roughly 6 percent of the Earth's land surface and are most common in temperate and boreal regions, wetlands perform a wide range of ecosystem functions, many of consequence on a global scale. Most of these functions are related either directly or indirectly to the activities of the flora and fauna living in sediments. Wetlands occur where saturation or inundation often produces anaerobic sediments, limiting rooted plant diversity to only those species adapted to anoxic conditions (Ewel 1997; Brinson & Malvarez 2002). Seasonal and interannual patterns of hydrologic regime and water source (rain, groundwater, and/or riverine surface water) govern many of the characteristics of wetland ecosystems, including species diversity and primary productivity. Freshwater wetland types include wet meadows, fens, bogs, lake margins, floodplain forests and bottomland swamps, tropical peat swamps, and extensive boreal peatlands. All wetlands are flooded long enough to influence the types of biota able to inhabit the site and the character and rate of biogeochemical processes. The global diversity of wetlands derives from regional and local differences in hydrologic regime (especially duration of flooding but also water residence time and water chemistry), physical factors such as fire and storms, unique characteristics of the plant species inhabiting those wetlands, and the influence of the animals that visit and live in them.
These many types of wetlands often are connected to surface and subsurface waters. Their ecosystem services include cycling of nutrients, breakdown of organic matter, and filtering of sediments that otherwise would enter rivers (Naiman & Decamps 1997; Keddy 2000). Yet, these critical habitats are being lost at a rapid rate despite their recognized values and legal standing (Dahl et al. 1991; Bedford 1999; Brinson & Malvarez
2002). Threats to rivers, floodplains, and lakes are also increasing (see Giller et al., Chapter 6) and are likely to result in loss of their essential ecosystem services.
The draining of wetlands and other threats to freshwater ecosystems have given rise to local and regional programs aimed at reducing their loss and restoring them to natural levels of diversity (Zedler 2000; Mayer & Galatowitsch 2001). In some cases new wetlands are constructed in other areas to attempt to offset the loss of natural wetlands (Moshiri 1993; Kladlec & Knight 1996). These constructed wetlands for "mitigation banking" can provide some ecosystem services, but often lack the biodiversity as well as the hydrologic regime that characterize natural ecosystems. Successful management for the sustainability and reliability of ecosystem services remains uncertain.
Provisioning Service. Riparian wetlands often have higher concentrations of microorganisms, insects, and animals than adjacent ecosystems (Naiman & Decamps 1997), and in arid regions they may be the only forested natural vegetation, thereby providing valuable habitat for arboreal species (National Research Council 2002). Many terrestrial animals, both vertebrates and invertebrates, use wetlands during some portion of their lives, and 50 percent of the 800 species of protected migratory birds in America rely on wetlands for habitat and food resources associated with benthic production of invertebrates and aquatic plants (Wharton et al. 1982). For example, 50 to 80 percent of the duck populations in North America are produced in north-central prairie potholes. These ecosystems provide hunters with significant recreational opportunities of economic importance (Batt et al. 1989). Waterbirds use a range of habitats including ponds, swamps, lagoons, mudflats, estuaries, embayments, and open shores of lakes, rivers, and reservoirs. Wetlands flooded to average depths of 15 to 20 cm (fringe and depressional wetlands) accommodate the greatest richness and abundance of birds (Taft et al. 2002).
Beavers play an important role in wetland landscapes as ecosystem engineers, creating a tremendous expansion of wetlands that otherwise would not have existed. Beaver harvests have averaged 400,000 pelts per year over the past century in North America (Novak et al. 1987). Alligators are also harvested for their pelts and meat, generating over US$16 million in a single year in the state of Louisiana, USA (Mitsch & Gosselink 2000). Crayfish aquaculture has also become an important use of natural and created shallow marshes in North America, northern Europe, and Australia in recent years. Pro-cambarus clarkii accounts for about 90 percent of the 60—70,000 tons of crayfish cultured annually for food in North America (Huner 1995).
Nearly all commercially harvested freshwater fish and shellfish species depend on fringe or riverine wetlands at some life stage (typically for spawning or for nursery habitat). Anadromous fishes are less reliant directly on freshwater marshes, but fry may use riverine marshes for protection. Plant foods are harvested from fringe, riverine, and depressional wetlands as well as from extensive peatlands. For instance, berries from boreal peatlands are an important and nutritious part of the diet typical of high-latitude human populations (Usui et al. 1994). The worldwide average annual harvest of blue berries (Vaccinium myrtillus) was 157,128.6 million tons (1990-2002), with approximately 42,000 ha in production (FAO 2003). The total wild berry harvest in Finland can be as high as 109 kg per season for a market value of more than US$240,000 (Wal-lenius 1999). Rice production in managed wetlands plays an important role in world nutrition and in the global economy. About 596 million tons of rice are produced each year (86 percent of this is consumed by human populations), harvested from 1.6 million km2 of wetlands (IRRI 2000).
Wetland timber is harvested for pulp and building materials; peat (partially decomposed organic material) for fuel and horticultural soil amendment; and herbaceous vegetation from marshes for livestock fodder, fuel, fiber, and other products. Harvesting may require lowering of water tables to facilitate access to and removal of materials, which may permanently alter species composition. Peat harvesting is often viewed as renewable, but recovery may take centuries or more. Peatlands cover 420 million ha globally, with the most extensive habitats located in Russia and Canada. Peat is used as fuel to generate electricity or for conversion to methanol or industrial fuels (Rydin et al. 1999; Mitsch & Gosselink 2000). It may also be used to remove toxic materials and pathogens from wastewater and sewage (Jasinski 1999). Wetland meadows of many kinds are used for harvesting fodder and grazing livestock throughout the world. In Scandinavia, wet meadows bordering lakes and rivers are some of the most productive areas for the production of livestock fodder (Nilsson 1999; Rosen & Borgegard 1999).
Supporting Service. Wetlands can recharge local and regional shallow groundwater water systems; small wetlands can be very important locally (Weller 1981). Many wetlands may also improve water quality by removing organic and inorganic materials from inflowing waters. Wetland vegetation takes up and stores nutrients and some toxic compounds, thereby removing them from rapid cycling. Where water levels fluctuate, microbial denitrification can reduce nitrogen loads.
Waste processing is a service most often attributed to wetlands, although it is generally restricted to a few kinds of wetlands that can treat only certain wastes under specific conditions. Generally, riverine and fringe wetlands treat non-point-source pollution, such as from agricultural fields, either directly through the uptake of nutrients, chemicals, and metals, or indirectly through the chemical transformation and processing of toxic compounds. For example, the freshwater tidal marshes of the Hudson River retain nutrients and result in denitrification when properly managed (Zelenke 1998). Depressional wetlands and extensive peatlands can substitute for tertiary wastewater treatment (e.g., Odum 1984; Ewel 1997), but the lack of control over waste processing has made construction of artificial wetlands more attractive (Ewel 1997). Wetlands created for further treatment of secondary sewage from major cities can remove up to 97 percent of the nitrogen delivered to them through a combination of uptake by plants and through denitrification (Costa-Pierce 1998).
The ability of wetlands to process wastes effectively depends on the rates of nitrogen, iron, manganese, sulfur, and carbon transformations that occur under increasingly low oxygen conditions in the sediments. Although wetlands maintain the widest range of oxidation-reduction reactions of any ecosystem, effective waste processing depends on appropriate ratios of many compounds. Overloading the system can compromise ecosystem functions. Waste processing and biological fixation of nitrogen relies on microbes such as Azotobacter, Clostridium butyricum, Rhizobium in root nodules, and cyanobacteria. Sediment-dwelling fauna affect surface and subsurface flows of water as well as stimulate microbial activity, even to the extent of changing the entire nature of a wetland. Beavers dam rivers, creating ponds and fringe wetlands, and alligators excavate cavities in wetlands in karst regions, such as the Florida Everglades (United States), facilitating the concentration of fish in patches of swamp wetlands during dry seasons.
Large expanses of wetlands (extensive peatlands in particular) are believed to affect global climate through the alteration of carbon dioxide and methane cycles. Burning peat as fuel further increases production of greenhouse gases such as carbon dioxide. Current global warming trends are likely to result in increased atmospheric trapping of greenhouse gases, in part because of the release of methane from boreal peat bogs. Wetlands contribute from 33 to 50 percent of the total annual methane production per year (100 teragrams; Whiting & Chanton 1993), mostly from boreal peatlands but approximately 25 percent from tropical and subtropical wetlands as well.
Cultural Services. Recreation such as bird watching, boating, fishing, and hunting are ecosystem services provided by many freshwater food webs that are supported by ben-thic organisms. In some areas, the recreational catch and value to the economy of recreational fishing outweigh the commercial catch because recreational fishermen spend nearly five times more per fish caught than commercial fishermen (DeSylva 1969). In South America, for example, the Pantanal provides many opportunities for ecotourism and recreational fishing in this enormous tropical wetland (approximately the size of the state of Florida). Its basin includes approximately 138,000 km2 in Brazil and 100,000 km2 in Bolivia and Paraguay (see Giller et al., Chapter 6). For four to six months of most years, some 70 percent of the land is inundated. Hunters, fishermen, and conservationists travel from all over the world to view and to exploit this exceptional biodiversity (Moraes & Seidl 1998). During the dry season, this wetland becomes a savanna used for grazing large herds of cattle.
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